It is commonly known that freshly cut fruits turn a brownish color when exposed to the air for a number of minutes. This is due to the action of the enzyme polyphenoloxidase. It is also well known that the formation of the brown color can be prevented or slowed by chilling the fruit, boiling it or rubbing it with lemon juice (which contains ascorbic acid). Explain why

Question

It is commonly known that freshly cut fruits turn a brownish color when exposed to the air for a number of minutes. This is due to the action of the enzyme polyphenoloxidase. It is also well known that the formation of the brown color can be prevented or slowed by chilling the fruit, boiling it or rubbing it with lemon juice (which contains ascorbic acid). Explain why

each of these methods works to slow down the browning process.

Answer 1

Chilling reduces the rate of the reaction.

Boiling removes the enzyme.
Oxygen reacts with both the enzyme and the citric acid but it reacts with citric acid first and will keep the browning away as long as the lemon juice is in excess.

Answer 2

Chilling the fruit: When fruits are chilled, the cold temperature inhibits the activity of the enzyme polyphenoloxidase. Enzymes are proteins that facilitate chemical reactions in living organisms. Lowering the temperature slows down the rate at which enzymes function, including the polyphenoloxidase enzyme responsible for browning. This is why refrigerating or chilling fruits can help delay the browning process.

Boiling the fruit: Boiling fruits denatures the enzyme polyphenoloxidase. Denaturation refers to the change in protein structure, typically caused by heat, which leads to the loss of its functionality. Heating the fruit disrupts the structure of the enzyme, rendering it ineffective in catalyzing the browning reaction. Therefore, boiling the fruit for a brief period can prevent or significantly slow down the browning process.

Rubbing with lemon juice (containing ascorbic acid): Lemon juice contains ascorbic acid, also known as vitamin C. Ascorbic acid acts as an antioxidant and inhibits the browning reaction by reacting with oxygen. Browning occurs when polyphenols in fruits react with oxygen in the air. Ascorbic acid, when applied to the fruit's surface, chemically reacts with the oxygen before it can react with the polyphenols, thus preventing or slowing down the browning process. Rubbing the fruit with lemon juice effectively provides a protective barrier against oxygen-mediated browning, thanks to the presence of ascorbic acid.